How to store electrical energy as heat

An idealized model for a system that would store large amounts of electrical energy by heating a tank of fluid has been developed by a physicist in Germany. The model is based on the concept of pumped heat electricity storage (PHES), which is a family of energy-storage technologies being developed worldwide to store electricity generated by intermittent sources such as wind turbines or solar panels. This latest research could help boost both the energy and cost efficiencies of these storage systems.

Large-scale stores

Renewable energy sources such as wind and solar do not produce energy at a constant rate and as a result engineers are developing large-scale energy-storage methods that can hold excess energy for use when the wind is not blowing or when the Sun is not shining. However, creating efficient storage systems is proving difficult as André Thess of the Ilmenau University of Technology points out in a recent paper in Physical Review Letters. Today, two techniques are used: pumped hydro storage (PHS) and compressed-air energy storage (CAES). Both, however, can be very difficult to implement. PHS needs kilometre-sized, elevated water reservoirs containing nearly 10 million cubic metres of water, while the CAES method involves finding or creating huge underground caverns.

PHES, on the other hand, is much simpler – electricity from a source such as a solar or wind farm is used to run a heat pump. The pump heats water stored in a large tank (normally about 100,000 cubic metres in volume) and then, when needed, the heated water is sent to a heat engine and electricity is produced. A heat pump, rather than an electric heater, is used to heat the water because it makes the whole process much more efficient. Heat pumps are designed to move thermal energy in the direction opposite to that of spontaneous heat flow and so use much less energy than would be needed to generate the heat with an electrical heater.

Optimized storage system

While this sounds great in theory, Thess points out that no large PHES system exists today and therefore the actual efficiency of such systems is still unknown. While other groups have proposed PHES systems that use everything from water, molten salt and liquid metals at various temperatures, predicting and comparing the performance of such systems has proved to be very difficult. The problem is that there are too many parameters involved; to overcome this, Thess has developed a simple thermodynamic model that can predict the efficiency of a PHES system as a function of the temperature of the thermal energy storage at maximum output power.

In his model, Thess assumes that the heat engine is optimized for maximum power – meaning that it produces electricity as quickly as it can – but not at maximum efficiency. By doing so, the efficiency of an entire cycle of storing and retrieving energy can be described by the ratio of the storage temperature to the ambient temperature of the surroundings.

So, for example, a PHES system that heats water at 20°C to 60°C would have an efficiency of about 38%. Thess says that the efficiency could be increased by increasing the storage temperature – which would involve using storage fluids other than water. However, he points out that water-based systems would be cheaper to build. With regard to established technologies, Thess's analysis suggests that for storage temperatures above 400°C, PHES would be more efficient than CAES.

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11 comments

Storage at modest scale

For the intermittent PV and wind renewable electrical energy (available for about 22% to 30% of the time per day) the storage is the basic need. Here the combination of the "heat pump" to store it as heat in an appropriate fluid and the reuse of this heat to reproduce the electricity via a heat engine with a good coversion efficiency, is good step, because this method can be used at a relatively modest scale unlike the other proposed tecniques.

Heat pump to store and recover energy

While I agree that you could use renewable energy to pump heat for storage, I dont see how you can recover that energy as electricity with any reasonable efficiency over a small differential like 20-60 degrees C. Seems to me the 2nd law of thermodynamics might beat you.

Sabatier

Let's just go with the Sabatier reactor and quit all the waste of time, pick it, vote yes, dictators order it, quit bulls..ing around with all the microorganisms. Pick et and go with it, like large (mass scale)solar farms, move on. Attempt to help the possible.

Storing energy

When thw Sun is not shining and the wind is not blowing, we could burn the hidrogen we have obtained decomposing water with the excess energy we obtain when the sun is shining and/or the wind is blowing. And injecting new oxigen to the atmosphere!

When thw Sun is not shining and the wind is not blowing, we could burn the hidrogen we have obtained decomposing water with the excess energy we obtain when the sun is shining and/or the wind is blowing. And injecting new oxigen to the atmosphere!

To get the stored energy back you have to burn maximum of obtained hydrogen gas to be effective. You need the same amount of oxygen you have produced to do that. So you won't get any new oxygen.

True

When thw Sun is not shining and the wind is not blowing, we could burn the hidrogen we have obtained decomposing water with the excess energy we obtain when the sun is shining and/or the wind is blowing. And injecting new oxigen to the atmosphere!

To get the stored energy back you have to burn maximum of obtained hydrogen gas to be effective. You need the same amount of oxygen you have produced to do that. So you won't get any new oxygen.

True, you don't get any new oxigen, but you don't use any new oxigen as it happens when you burn something to grt energy